Method of producing silicon and electrolytic cell therefor



Oct. 8, 1968 D. BARAKAT ET AL METHOD OF PRODUCING SILICON AND ELECTROLYTIC CELL THEREFOR Filed July 50. 1965 F'lC-ll.

INVENTORS: DLAWAR BARAKAT HANS KELLER BY W LFMMQWN QM THEIRYATTORNEYS United States Patent s 8 Claims. (Cl. 204-60) This invention relates to the electrolytic production of silicon from silica and, more particularly, to a method of and cell for producing silicon electrolytically, in a pure state.

It has been recognized that silicon can be obtained by electrolysis of a fused or molten bath of cryolite containing silica. The purity of the silicon is dependent to a large extent upon the purity of the electrolytic bath or electrolyte. Electrolytic reduction and deposition must be carried out above the melting point of cryolite, e.g., in a range between about 970 C. and 1050" C. Due to the high temperature involved, it is difficult to find materials for the construction of a suitable electrolytic cell which do not react with and contaminate the molten cryolite with resulting contamination of the silicon deposited electrolytically.

Graphite is recognized as a material which does not react chemically with fluorides or cryolites. However, it is difiicult to eliminate oxygen from the type of electro lyte used and from the atmosphere above the electrolytic bath, and when oxygen is present, combustion of the graphite takes place which causes deterioration of the cell and contaminates the bath with the combustion products of the graphite and/ or impurities contained in the graphite. Accordingly, cells formed of or having the inner walls of the cells lined with graphite deteriorate rapidly and have a very short operating life.

In order to avoid deterioration of cells using molten cryolite with or without additions of fluorides as an electrolyte, it has been suggested that such cells can be lined with fluoride-resistant materials such as boron nitride or silicon carbide with silicon nitride as a bonding agent. However, all these materials are also a source of contamination of the electrolyte and thus reduce the purity of the electro-deposited silicon In accordance with the present invention,'cells are formed of material which resists the attack of molten cryolite during the electrolysis and moreover even if the material is attacked by the molten cryolite, it will not contaminate the bath in any undesirable manner. More particularly, in accordance with the present invention, a cell is provided which is formed of pure silica granules or sand packed in a jacket or container and having a cavity therein adapted to receive the molten electrolyte and an anode and a cathode by means of which silica is reduced to silicon and deposited on the cathode in the form of pure or substantially pure silicon. Inasmuch as the electrolyte contains silica as an essential component to enable the electro-deposition of silicon by reduction of the silica to silicon, the use of the silica cell will not introduce any contaminants so long as the silica or sand of which the cell is composed is of a high degree of purity. While the cell made of finely divided silica or sand does not have a great deal of initial wall strength, the molten electrolyte in the cavity will penetrate or seep into a zone of substantial thickness surrounding the cavity and due to the progressively decreasing temperature from the cavity to the outside of the cell, the electrolyte will solidify in a zone spaced between the cavity and the outside of the silica body with the result that a barrier wall is formed of a cemented mixture of the solidified electrolyte and 'ice silica which is form-retaining and provides other advantages as will be explained hereinafter.

For a better understanding of the present invention, reference may be had to the accompanying drawings, in which FIGURE 1 illustrates schematically and in cross-Section a typical cell embodying the present invention; and

FIGURE 2 is a plan view of the cell.

Referring now to the drawings, a typical cell used, for example, on a laboratory scale, includes a cylindrical container or jacket 10 formed of metal, such as, for example, steel, silica brick, or any other suitable material, and supported upon legs 11 or otherwise, as may be desired. In fact, the cell may be formed in a cavity in the earth or the floor of a building. Within the container is rammed a body 13 of finely divided pure silica or pure sand and shaped in its midportion to form a regularly or irregularly shaped cavity 14 therein about 23 inches in diameter and about 15 inches deep. A suitable procedure for forming the cell is to fill the bottom of the container with rammed sand and then insert a shell or tubular mold spaced from the wall of the container and fill the space between the shell and the container wall with sand. The space Within the shell can be filled with finely divided solid cryolite or cryolite and silica. After the bottom and walls of the mold are formed and the space is filled with solid electrolyte, the shell is removed and the top of the container is filled with sand or silica to form a non-removable cover 12 which is a continuation of the body 13. This cover is made out of silica or sand to which was added 2% sodium silicate as a bonding agent to give it the necessary rigidity. This cover 12 is provided with a series of openings 15, 16, 17, 18, 19 and 20 for receiving anodes 21, 22 and 23 and cathodes 24, 25 and 26 in the form of the invention illustrated.

The electrodes can be positioned in the cell during its formation so that the anodes are embedded in but movable relative to the cover and extend downwardly into the finely divided solid electrolyte. Larger openings are formed for the cathodes to enable their withdrawal.

Removable covers 35, 36 and 37 are made of regular refractory bricks and cover the cathodes openings 16, 17 and 20. For larger cells, a greater number of pairs of anodes and cathodes may be provided and for smaller cells, a relatively smaller number of anodes and cathodes may be provided. In the type of cell illustrated, the direct current which is supplied to the anode and cathode or anodes and cathodes is insufficient to maintain the temperature of the cell within the desired operating range, i.e., 970 C. to 1050 C., and for that reason, other electrodes 27 and 28 may extend through the cover 12 into contact with the electrolyte E in the cell for passage of alternating current therethrough to supply additional heat. In larger commercial production cells, alternating current heating is not required and these electrodes can be omitted.

Referring now to FIGURE 1, the anodes and cathodes 21 to 26 may be formed of pure graphite and are connected by means of suitable bus bars or conductors 29 and 30 to a source of direct current. In order to melt the finely divided electrolyte upon starting the cell, the electrodes are short-circuited by means of graphite bricks or strips which may be placed in the cell during its formation and filling with electrolyte. A.C. or DC. applied to the short-circuited electrodes will heat and melt the electrolyte and bring the cell up to operating temperature. The electrodes then are withdrawn from contact with the short-circuiting strips or bricks and, if desired, the bricks or strips can be removed through the openings 16, 17 and 20. The above-described procedure is more satisfactory than charging the cell with'molten electrolyte for the reason that the molten electrolyte damages the cell walls as it is introduced into the cell and also freezes and solidifies when the cell walls and bottom are cold.

As indicated above, the electrolyte is composed principally of cryolite and it may also contain silica in an amount between about 3% and depending upon the operating temperature of the cell. Although the cell can operate at a temperature as high as 1400 C., the normal operating temperature is about 1040 C. At this temperature, the electrolyte will contain approximately 10% silica. At a lower temperature, e.g., 970 C., the electrolyte contains 3% silica which is added initially to the electrolyte and is dissolved therein.

When the electrolyte is in a molten condition, it flows into and permeates the silica or sand throughout a zone 32 which is proportional to the temperature of the electrolyte as it penetrates outwardly from the cavity. The electrolyte solidifies at 950 C. and as a consequence, when the temperature of the electrolyte permeating the sand or silica is reduced to that temperature, it solidifies and forms, in effect, a barrier wall and also cements together the sand or silica grains. The thickness of the zone of penetration 32 around the cavity 14 will vary, depending upon the temperature of the cell and the electrolyte therein, the thickness of the barrier zone decreasing at higher temperatures and increasing at lower temperatures. At a temperature of about 970 C., the wall is practically an isotherm,

Under normal operating conditions, that is electrolysis at 1040 C., practically no attack on the cell by the electrolyte occurs because the electrolyte is saturated with silica which is introduced into the bath from above, as needed. Saturation of the electrolyte produces a separation of the electrolyte into two distinct liquid phases, namely, an upper phase E1 which is rich in cryolite and contains 3 to 10% silica dissolved therein, depending upon the temperature of the bath, and a lower phase E2 of very much smaller volume which contains up to 80% silica and usually between 50% and 80% silica, also depending upon the temperature of the bath. The lower phase E2 assures saturation of the upper phase E1 with silica. For example, when the concentration of silica in the upper phase E1 decrease, due to deposition of silicon on the cathode or cathodes, the necessary amount of silica to bring up the level of silica concentration in its upper layer is dissolved from the lower phase E2. The silica added from the top of the cell goes directly to the lower liquid phase E2 when phase E1 is already saturated with silica. Thus, by supplying silica directly from the top of the cell, little or no withdrawal of silica from the cell wall itself occurs and an equilibrium condition is established between the saturated condition of the electrolyte and the silica or sand in contact with the electrolyte. In normal operation, the lower phase E2 is about one-tenth the total volume of the electrolytic bath E. The temperatures of the two layers E1 and E2 vary between 970 and 1050 C. As mentioned above, the temperature of the barrier zone 32 is always below 970 C. If this barrier zone 32 tends to move outwards due to an increase in temperature, the silica or sand of the barrier zone 32 goes into solution and is dissolved in layer E2. Thus, it is possible to maintain a state of equilibrium in the cell, by controlling the temperature and the silica concentration which will prolong the cell life indefinitely.

For a cell of the type illustrated, a direct current at about 7 volts and 400 amperes is supplied across the cathodes and anodes. The anodic current density can be as high as 150 amperes per square decimeter. A 100 amperes per square decimeter anodic current density is adequate for most purposes. As for the cathodic current density, it varies with the size of the cathodic ball B and is difiicut to determine. The temperature of the cell is maintained between 970 C. and 1050 C. and, in fact, close to 1040 C. by supplying additional heat with the alternating current electrodes 27 and 28 at about volts and a 4 200 amperes. The current efiiciency for the cell illustrated is about 90%.

From the foregoing, it will be clear that a cell in which the cell cavity has walls of pure silica reduces to a marked extent the contamination of the silicon and, in fact, the only source of contamination is the anodes themselves which, as would be expected, are consumed during the operation and, accordingly, must be fed slowly and continuously into contact with the electrolyte by means of a suitable type of electrode support. By forming the anodes of pure graphite, contamination is reduced to a minimum.

The silicon collects in the form of a pasty ball B or shell on the cathode or cathodes and is strip ed from the cathode periodically. The silicon is separated from the electrolyte by leaching out or subliming off the electrolyte which clings to and is carried over with the ball B or shell. The silicon crystal obtained are further treated, as may be required, for industrial and electronic purposes.

When the electrolyte is maintained in a saturated condition by the addition of silica to it, on the one hand, and on the other hand by adequate control of the operating temperature, the cell life is prolonged indefinitely due to the fact that little or no silica is dissolved from the cell itself during the electrolytic reaction.

It will be understood that the formation of the cavity for receiving the electrolyte may be accomplished in various ways, such as, for example, by ramming the container 10 with sand or packing the sand in and scooping out a cavity or by bonding the sand together with suitable binders such as cryolite in order to avoid introduction of contaminants into the electrolyte adversely affecting the purity of the silicon deposited from the electrolytic bath.

Accordingly, the example of the invention given herein should be considered as illustrative and not as limiting the scope of the invention as defined in the following claims.

We claim:

1. A method of producing silicon comprising introducing an electrolyte comprising molten cryolite into a cavity in a mold composed principally of finely divided silica, dissolving approximately 3% to 10% silica in said molten cryolite and maintaining said molten cryolite and dissolved silica at a temperature between about 970 and 1400 C., passing a direct current through said electrolyte between an anode and a cathode immersed in said electrolyte to reduce said dissolved silica to silicon and deposit said silicon on said cathode.

2. The method set forth in claim 1 comprising introducing additional silica into said electrolyte as said silicon is deposited on said cathode to establish substantial equilibrium between the silica dissolved in said electrolyte and said silica of said mold.

3. The method set forth in claim 1 comprising supplying heat to said electrolyte to maintain it between about 970 and 1400 C. by passing alternating current through said electrolyte between a pair of electrodes immersed in said electrolyte.

4. The method set forth in claim 1 in which said electrolyte penetrates into said mold around said cavity and solidifies therein to form a barrier layer spaced from said cavity in equilibrium with said electrolyte.

5. The method set forth in claim 1 in which said electrolyte is separated into an upper liquid phase containing approximately 3% to 10% silica and a lower liquid phase containing up to silica.

6. A cell for producing silicon from silica comprising a container, a body principally of finely divided silica in said container, a cavity in said body of silica for receiving a molten cryolite electrolyte, at least one pair of electrodes extending into said cavity and a cover for said container.

7. The cell set forth in claim 6 in which said cell is devoid of exterior heating means to enable said electrolyte to penetrate into and solidify in said body around said 5 cavity to form a barrier layer in thermal equilibrium with said molten electrolyte.

8. A method of producing silicon comprising forming a cavity in a body composed principally of silica, filling the cavity at least partially with a finely divided, solid electrolyte of the class of cryolite or cryolite and silica, passing electrical current between interconnected electrodes in contact with said electrolyte to melt said electrolyte and heat it to between about 970 C. and 1400 C., disconnecting said electrodes from each other, maintaining in said electrolyte between about 3% and 10% of dissolved silica and passing a direct current from one electrode to another electrode immersed in the molten electrolyte to reduce said dissolved silica to silicon and deposit said silicon on one of said electrodes.

References Cited UNITED STATES PATENTS 920,893 5/1909 Blackmore 204-245 XR 1,980,378 11/1934 Burgess 204243 XR 2,892,763 6/1959 Stern et al 204-246 XR 3,254,010 5/1966 Monnier et al. 204-60 XR JOHN H. MACK, Primary Examiner.

D. R. VALENTINE, Assistant Examiner. 

1. A METHOD OF PRODUCING SILICON COMPRISING INTRODUCING AN ELECTROLYTE COMPRISING MOLTEN CRYOLITE INTO A CAVITY IN A MOLD COMPOSED PRINCIPALLY OF FINELY DIVIDED SILICA, DISSOLVING APPROXIMATELY 3% TO 10% SILICA IN SAID MOLTEN CRYOLITE AND MAINTAINING SAID MOLTEN CRYOLITE AND DISSOLVED SILICA AT A TEMPERATURE BETWEEN ABOUT 970* AND 1400*C., PASSING A DIRECT CURRENT THROUGH SAID ELECTROLYTE BETWEEN AN ANODE AND A CATHODE IMMERSED IN SAID ELECTROLYTE TO REDUCE SAID DISSOLVED SILICA TO SILICON AND DEPOSIT SAID SILICON ON SAID CATHODE. 